There are many applications where binary information generated by circuitry of a first logic family is to be utilized by circuitry of a second logic family, and where the logic high and logic low voltage levels for circuitry of the two logic families are different. For example, in a multibit digital to analog converter, the input digital information may be generated by TTL logic while the logic for the converter circuit may be CMOS. A TTL to CMOS converter is thus required as a front end to the D/A converter circuit.
A number of voltage level conversion circuits, and in particular TTL to CMOS conversion circuits, currently exist. For example, in its simplest form, the circuit might consist of a CMOS inverter configured such that its switching threshold is at a voltage level roughly midway between the logic high and logic low voltage levels for the logic family providing inputs to the circuit followed by a second output stage inverter. Ideally, this would assure that the output voltage level will always be at the same logic state as the input voltage level. However, in practice, it is difficult to design such circuits so that they provide true outputs over extended temperature ranges, such as the full temperature range provided in military specifications, and over the anticipated specification ranges for the various components and other circuit parameters utilized. Thus, use of such circuitry may require a relaxing of performance specifications which is unacceptable in many applications.
Other circuits for performing the conversion function utilize differential input comparators with threshold levels set by a voltage reference circuit such as two diodes in series. This technique requires the use of bipolar transistors, requires extra bias circuitry for the comparator and diodes and, owing to the use of a comparator, has a relatively long delay time which limits the clock rate at which the circuit may be utilized. Other circuits for performing the conversion function, such as that shown in U.S. Pat. No. 4,791,318, issued Dec. 13, 1988 and assigned to the assignee of this application, involve complicated closed loop control, and thus require a substantial amount of circuitry. These devices are thus relatively expensive.
A need therefore exists for a voltage level conversion circuit, and in particular a TTL/CMOS conversion circuit, which is both simple and inexpensive while still providing high performance over extended temperature ranges and extended variations in component and other circuit parameters. In particular, the circuit should assure that the output logic level always corresponds to the input logic level regardless of temperature and parameter variations, so long as such variations are within relatively wide specifications set for such parameters in the design. Where multibit values are being converted, requiring a multistage converter, it is also desirable that the converter be designed so as to minimize power requirements.